JP-A-H07-87734 discloses a main power control unit that, in a standby mode (power saving mode), stops oscillation of an output transformer of a switching power supply and then a secondary battery supplies power to the main power control unit such that power is saved.
Recently, it has been required to further save power for a switching power supply. In the technology disclosed in JP-A-H07-87734, even when the secondary battery is charged in the standby mode, the output transformer outputs 24V, and thus power is wastefully consumed. For this reason, a method using an electric double-layer capacitor or the like instead of the secondary battery can also be considered. However, in a case where a standby period is long, it is necessary to make the output transformer primarily oscillate to charge the electric double-layer capacitor, and thus it is desired to further save the power of the power supply system in the power saving mode.